Using epidemiologic methods, we propose to evaluate the influence of duration of oral anticoagulant therapy following and episode of pulmonary embolism or deep vein thrombophlebitis on risk of recurrent thromboembolism following cessation of therapy and of major hemorrhagic events during therapy. This study is designed both to help answer this important clinical question and to demonstrate the validity of nonexperimental approaches to evaluating drug efficacy. In the proposed study, we will identify the 2,100 individuals who were admitted to any of thirteen medical facilities in the period 1971-1978 with pulmonary embolism or deep vein thrombophlebitis in the absence of an immediately predisposing condition. The medical records of these individuals will be reviewed in detail to establish the firmness of the initial diagnosis of thromboembolism, to determine the duration of oral anticoagulant therapy following the initial event and to ascertain the occurrence or recurrent thromboembolism and of major hemorrhagic events. Statistical life-table techniques will be used to compare the probability of recurrent thromboebolism following therapy in relation to duration therapy, adjusting for variables such as age and blood pressure which may also be related to risk of recurrence. We will also examine the covariate adjusted probability of hemorrhagic complications during therapy.